An in-born appetite for salt, more specifically sodium chloride or NaCl, is found in many mammals, including humans. Historically, salt was considered a valuable commodity to human populations living far from the sea. Soldiers in the army of ancient Rome received an allowance of salt called a salarium. Later on, the allowance was changed to money to buy salt; hence, the word "salary" and the expression "not worth his salt."
The importance attached to salt or, more accurately, sodium, is not without a biological basis. Sodium accounts for approximately 90% of the extracellular cations in humans, thereby making it the most important factor in determining the volume and concentration of blood and extracellular fluid. In this way, sodium also affects blood pressure. Many studies, including Intersalt, a survey of over 10,000 people in 32 countries, have linked high salt ingestion with high blood pressure. The average American consumes nine grams of sodium per day; the average Japanese, fourteen. Concern over high blood pressure, or hypertension, and its cardiovascular consequences, thus has prompted many people to limit their intake of salt and other sodium-containing products.
However, the "perfect" salt substitute has eluded many researchers, mostly because of the specificity of the mechanism for salt taste perception. (Erickson D. "Trick of the Tongue: A unique mechanism of taste means no substitute for salt." Scientific American, pages 80-81, May 1990). According to current theory, the taste of salt begins when sodium ions pass through specialized pores, or sodium ion channels, in taste bud cell membranes. The influx of sodium ions causes the taste bud cells to depolarize, triggering the release of neurotransmitters which excite the nerves carrying the salt message to the brain. (Heck GL et al. "Salt Taste Transduction Occurs Through an Amiloride-Sensitive Sodium Transport Pathway." Science 223: 403-4 (1984)).
So far, only sodium and lithium have been shown to pass through this channel, thus limiting the possibilities for "true" salt alternatives. Though potassium does not pass through the sodium ion channels, it has been used, in the form of potassium chloride (Kcl), in the "lite salt" mixtures currently sold. However, the utilization of potassium chloride as a salt substitute is limited by the bitter aftertaste imparted by the compound.
Others have looked to so-called "salty" peptides as alternatives to salt, particularly derivatives of ornithyltaurine and ornithyl-.beta.-alanine. (Tamura M. et al. "An Enhancing Effect on the Saltiness of Sodium Chloride of Added Amino Acids and Their Esters." Agric. Biol. Chem. 53(6):1625-1633 (1989); Seki T et al. "Further Study on the Salty Peptide Ornithyl-.beta.-alanine. Some Effects of Ph and Additive Ions on the Saltiness." J. Agric. Food Chem. 38: 25-29 (1990)). However, these compounds are not salty in the absence of Hcl. Moreover, their cost and difficulty of synthesis are expected to limit their utility as salt substitutes. (Worthy W. "New sweet, salty peptides synthesized." Chemical & Engineering News, pages 25-26 (Jan. 8, 1990)).
Knowledge of the mechanism of salt taste perception, coupled with the finding that lithium chloride is poisonous, has led some to search for salt enhancers, that is, substances which boost the "saltiness" of sodium-containing compounds, thereby permitting the use of lower levels of sodium chloride. However, until now, researchers likewise have been stymied in their quest for salt enhancers, with one chemosensory physiologist in Virginia recently reporting that he had spent 18 months screening food compounds without uncovering any good candidates. (Erickson D. "Trick of the Tongue: A unique mechanism of taste means no substitute for salt." Scientific American, pages 80-81, May 1990).